# this does 1000 requests to the URL. No problems here.
ab -n 1000 http://my-test-service.com

# this always does requests to the URL for 60 seconds. WRONG
ab -t 60 http://my-test-service.com

If you type ab without arguments you get a basic list of options including..

-t timelimit Seconds to max. wait for responses

Easy enough. What else could one possibly need to know to use this.

The man description of this option has a bit more info

-t timelimit
Maximum number of seconds to spend for benchmarking. This implies
a -n 50000 internally. Use this to benchmark the server within a fixed total
amount of time. Per default there is no timelimit.